Photodynamic Therapy (PDT) using photosensitive tumor localizing drugs and laser light is undergoing extensive basic, preclinical and clinical research. The therapeutic response obtained by PDT is a product of the photosensitizer concentration in the tissue and the light it absorbs. Physiological factors such as tissue O2 content, ph, and temperature may also play a role in determining the therapeutic response. It is the objective of this Phase-II project to further develop and test a Photodynamic Therapy (PDT) dosimeter system to improve the modality. This development will include (1) further refinement of a fiber optic isotropic probe for measuring the space irradiance in-vivo, (2) increasing the sensitivity of the fiber optic probe for simultaneously measuring the fluorescence of the photosensitizing drug in-vivo, (3) correlation of the measured parameters with biological response in a standardized animal model, (4) evaluation of detecting singlet oxygen emission at 1270nm as a dosimetry monitor and (5) preliminary evaluation of the usefulness of the instrument in patients undergoing PDT treatment for recurrent chest wall metastasis of breast cancer. In Phase-III the instrument will be further standardized, and controlled clinical trials undertaken to obtain FDA approval for commercialization.